Chapter 8 - Cell - Structure and Functions

WHAT IS CELL?

Cell is the basic building block of all living organism or the smallest structural or functional unit of an organism.

DISCOVERY OF CELL

• Robert Hooke in 1665 discovered the cell.
• He took thin cork slices of cork and observed them under microscope.
• He noticed block like structure that appeared like honey comb.
• Hooke coined the term 'cell' for each box.

THE CELL

• Basic structural unit.
• Have different designs, sizes and shape.
• Cells in living organism are complex unlike non-living bricks.

TYPES OF CELLS

1. Prokaryotic Cells - The cells having nuclear material without nuclear membrane are termed as prokaryotic cells. Examples: bacteria and blue green algae.

2. Eukaryotic cells - The cells having well organised nucleus with a nuclear membrane are designated as eukaryotic cells.

ORGANISMS SHOW VARIETY IN CELL NUMBER, SHAPE AND SIZE

• Millions of living organisms
• Have cells with different shapes and sizes.
• Their organs also vary in shape, size and number of cells.

NUMBERS OF CELLS

• Human body has trillions of cells which vary in different size and shapes.
• Organisms made of more than one cell are calledmulticellular organisms.
• An organism with billions of cells Starts life as a single fertilized egg cell.
• The fertilised egg cell multiplies by dividing process and the number of cells increases for development.
• The single-celled organisms are made up of single cell are called unicellular organisms.
• A single-celled organism, like amoeba, captures and digests food, respires, excretes, grows and reproduces.
• Similar functions in multi-cellular organisms are carried out by groups of specialised cells forming different tissues. Tissues, in turn, form organs.

SHAPE OF CELLS

• Generally, cells are round, spherical or elongated.
• Some types of cells are long and pointed at both ends exhibit spindle shape.
• Some types of cells are branched like the nerve cell or a neuron.
• Example- Spherical red blood cells of humans, Spindle shaped muscle cells, long branched nerve cell.
• Components of the cell are enclosed in a membrane. This membrane provides shape to the cells of plants and animals.

SIZE OF CELLS

• The size of cells in living organisms may be as small as a millionth of a metre (micrometre or micron) or may be as large as a few centimetres.
• Most of the cells are microscopic in size and are not visible to the naked eye.
• The smallest cell is 0.1 to 0.5 micrometre in bacteria. The largest cell measuring 170 mm x 130 mm, which is the egg of an ostrich.
• The size of the cells has no relation with the size of the body of the animal or plant.
• Size of cell is related to its function.

CELL STRUCTURE AND FUNCTION

• Each organ in the living system performs different functions such as digestion, assimilation and absorption. Similarly, different organs of a plant perform specific/ specialized functions.
  • Example: roots help in the absorption of water and minerals.
• Each organ is further made up of smaller parts called tissues (group of similar type cells performing a particular function.).

PARTS OF CELL

The basic components of a cell are cell membrane, cytoplasm and nucleus.

1. Cell Membrane

• The cytoplasm and nucleus are enclosed within the cell membrane, also called the plasma membrane.
• This membrane separates cells from one another and also the cell from the surrounding medium.
• Plasma membrane is porous and allows the movement of substance.
• This gives shape to the cell.
• An outer thick layer in cells of plants, called cell wall.
• This additional cell wall surrounding the cell membrane is required by the plants for protection against variations in temperature, high wind speed, atmospheric moisture, etc. They are exposed to these variations because they cannot move.
• Cells can be observed in the leaf peel of Tradescantia, Elodea or Rhoeo.

2. Cytoplasm

• The jelly-like substance present between the cell membrane and the nucleus.
• Various other components or organelles of cells are present in the cytoplasm. Like mitochondria, Golgi bodies, ribosomes, etc.

3. Nucleus

• Important component of the living cell.
• Generally dense and spherical organelle and located in the centre of the cell.
• Nucleus is separated from the cytoplasm by a membrane called the nuclear membrane (This membrane is also porous and allows the movement of materials between the cytoplasm and the inside of the nucleus.)
• Nucleus Consist of nucleolus and thread-like structures called chromosomes. These carry genes.
• Nucleus acts as control centre of the activities of the cell. The entire content of a living cell is known as protoplasm (includes the cytoplasm and the nucleus).
• Nucleus is the storehouse of genes. Without nucleus, cell can neither survive nor shows specialized activities.

COMPARISION OF PLANT CELL AND ANIMAL CELLS